The least expensive option is http://www.chefknivestogo.com/140grdistflp.html
I have and like this.http://www.chefknivestogo.com/xxcdiplandun.html
I set it on between the two kitchen sinks and gently run water over the plate as I flatten.
It flattens everything from Nubatama 150 to Shapton Glass 8K.
If you ever want to strop or free hand this set is a good value. http://www.chefknivestogo.com/stsetwiunhob.html